Mobility is nice to have but it is another red herring for ATCOs.
Moving from small Tower to small Tower is OK. But Good money is made in the large Centres , not in small towers.
And in ACCs, 3/4 of the licence is local OJT.
To convert an ACC controller to another ACC is a very lengthy process ( up to a year OJT) and you better do it before you're 35 to have good chances of requalifying . To convert a controller from a small TWR to a large ACC makes hardly no difference from Ab initio recruitment.
It will work for a few, sure , but you will find 95% will stay with their current employer. Even if they want to move en masse ( e.g Spain) all those above 40 will probably never be taken by anyone else.
Controllers are not like Pilots in that respect .